Method for protecting oil well casings



aired. drama METHOD FOR PROTECTING OIL WELL CASINGS No Drawing. Appiication August 7, 1953, Serial No. 373,041

5 Claims. (Cl. 166-1) This application is directed to a method for protecting casing in a well against corrosion.

In cased wells such as those for producing oil, gas or water, the annulus between the exterior of the casing and the Wall of the borehole may contain solutions which are highly corrosive to the steel casing. This fluid may be corrosive because it is a native formation water of high corrosivity or because it is in contact with corrosive salt beds or because the drilling fluid used in drilling the well conducts stray currents. In any event, the fluid in the annulus may be highly corrosive to the external surface of the steel casing, thereby causing early failure of the member and necessitating casing repair.

The present invention is directed to a method for suppressing or preventing such corrosion of the casing.

In accordance with the present invention, well casing is protected from corrosion by displacing fluid originally in the annular space between the casing and the wall of the borehole with a noncorrosive aqueous solution of water soluble alkylated cellulose ether. The solution used is sufficiently mobile to be pumped into position in the annulus and is capable of forming a gel within said annulus upon being retained quiescent therein. After the solution has been placed, it is maintained quiescent in said annulus until it forms a noncorrosive gel around the casing in the space between the casing and the wall of the borehole. The gel forms an impervious coating on the well casing, whereby contact with corrosive salt beds, corrosive native formation water and media of high conductivity is eliminated. The electrical conductivity of the coating is substantially negligible so that stray currents conducted down the casing from surface equipment are prevented from leaving the casing to conductive formations. This eliminates concentrated corrosion of the easing which results upon passage of current from the casing to highly conductive formations.

In carrying out the method of the present invention, a non-conductive aqueous solution of water soluble alkylated cellulose ether is formed. The solution is sufliciently mobile to be pumped into place and once placed will form a gel under the temperature conditions present in the well. It is preferred that the solution used have an absolute viscosity within the range of 100 to 4000 centipoises.

The preparation and use of aqueous solutions of water soluble alkylated cellulose ether is known. The most common water soluble alkylated cellulose ethers are termed methyl cellulose ether and ethyl cellulose ether, although each of these materials as used in commerce, is considered to be a mixture of ethers rather than a single ether. In addition, other alkylated ethers or mixtures thereof may be employed, such as propyl cellulose ethers and butyl cellulose ethers.

The viscosity of the aqueous solutions of such ethers is in general a function of the molecular weight of the ethers, of the concentration of the solution and of the temperature of the solution. In the practice of the present invention the viscosity of the solution may vary over 0 2,795,278 Patented June 11, 1957 a wide range and satisfactory results will be obtained. In

' the use of the solution it is essential that it be non-conductive, that it be sufficiently mobile to permit placement, as by pumping downwardly into the well and then upwardly from the bottom of the easing into the annular space between the casing and the wall of the borehole, and that it be capable of forming a gel within the well upon being retained quiescent in said annular space in the well.

Generally, it will be found preferable to use water solutions of alkylated cellulose ethers in the concentration range of .5 to 4%, said solution being capable of forming a :gel when retained in a quiescent state at a temperature range of 126 to 150 F. It is preferred to use such alkylated cellulose ethers whose 5% aqueous solutions have a viscosity range of to 4000 centipoises at 68 F. By way of specific example, a 2% by weight water solution of what is known as a medium viscosity type of methyl cellulose ether has a viscosity of 400 centipoises at 68 F. This solution is perfectly fluid at lower temperatures but gels at a temperature of approximately 122 F. to a very viscous or plastic mass.

Having fully described the method of the present invention, what is desired to be claimed is:

1. A method for protecting a casing in a well from corrosion including the steps of displacing corrosive fluid in the annular space between the casing and the wall of the borehole with a non-conductive aqueous solution consisting of water and water soluble alkylated cellulose ether sufliciently mobile to be pumped into place having an absolute viscosity within the range of 100 to 4000 centipoises at 68 F. and capable of forming a gel within the well upon being retained quiescent therein and maintaining said aqueous solution quiescent in said annular space between the casing and the wall of the borehole at a temperature in the range of 126 to F. to form a non-conductive gel, and then producing a fluid through said casing while maintaining said gel in said annular space.

2. A method for protecting a casing in a well from corrosion including the steps of displacing corrosive fluid in the annular space between the casing and the wall of the borehole with a non-conductive aqueous solution consisting of water and water soluble methyl cellulose ether sufficiently mobile to be pumped into place having an absolute viscosity within the range of 100 to 4000 centipoises for a 2% solution at 68 F. and capable of forming a gel upon being retained quiescent within the well and maintaining said aqueous solution quiescent in the annular space between the casing and the wall of the borehole at a temperature in the range of 126 to 150 F. to form a non-conductive gel, and then producing a fluid through said casing while maintaining said gel in said annular space.

3. A method in accordance with claim 2 in which said solution has a concentration within the range of .5 to 4%.

4. A method for protecting a casing in a well from corrosion including the steps of displacing corrosive fluid in the annular space between the casing and the wall of the borehole with a non-conductive aqueous solution consisting of water and water soluble ethyl cellulose ether sufliciently mobile to be pumped into place having an absolute viscosity within the range of 100 to 4000 centipoises for at 2% solution at 68 F. and capable of forming a gel within the well upon being retained quiescent therein and maintaining said aqueous solution quiescent in said annular space between the casing and the wall of the borehole at a temperature in the range of 126 to 150 F. to form a non-conductive gel, and then producing a fluid through said casing while maintaining said gel in said annular space.

5. A method in accordance with claim 4 in which said solution has a concentration within the range of .5 to 4%.

References Cited in the file of this patent UNITED STATES PATENTS 2,543,868 Prokap Mar. 6, 1951 2,582,909 Laurence Jan. 15, 1952 2,593,057 Savoy Apr. 15, 1952 2,742,090 Garrison Apr. 17, 1956 FOREIGN PATENTS Canada Nov. 29, 1949 OTHER REFERENCES Mills, R. Van A.: Protection of Oil and Gas Field Equipment Against Corrosion, Bulletin 233, Dept. of the Interior, Bureau of Mines, 1925, pp. 73-79. 

2. A METHOD FOR PROTECTING A CASING IN A WELL FROM CORROSION INCLUDING THE STEPS OF DISPLACING CORROSIVE FLUID IN THE ANNULAR SPACE BETWEEN THE CASING AND THE WALL OF THE BOREHOLE WITH A NON-CONDUCTIVE AQUEOUS SOLUTION CONSISTING OF WATER AND WATER SOLUBLE METHYL CELLULOSE ETHER SUFFICIENTLY MOBILE TO BE PUMPED INTO PLACE HAVING AN ABSOLUTE VISCOSITY WITHIN THE RANGE OF 100 TO 4000 CENTIPOISES FOR A 2% SOLUTION AT 68%F. AND CAPABLE OF FORMING A GEL UPON BEING RETAINED QUIESCENT WITHIN THE WELL AND MAINTAINING SAID AQUEOUS SOLUTION QUIESCENT IN THE ANNULAR SPACE BETWEEN THE CASINGAND THE WALL OF THE BOREHOLE AT A TEMPERATURE IN THE RANGE OF 126* TO 150*F. TO FORM A NON-CONDUCTIVE GEL, AND THEN PRODUCING A FLUID THROUGH SAID CASTING WHILE MAINTAINING SAID GEL IN SAID ANNULAR SPACE. 